1. Field of the Invention
The present invention relates to an interworking control between different parties during a communication connection. In particular, the present invention relates to a method and an apparatus usable for an interworking mechanism in a communication connection between parties of different networks when one of these parties intends to change an original media composition of the communication connection.
For the purpose of the present invention to be described herein below, it should be noted that                a terminal device may for example be any device by means of which a user may access a communication network; this implies mobile as well as non-mobile devices and networks, independent of the technology platform on which they are based; only as an example, it is noted that communication equipments operated according to principles standardized by the 3rd Generation Partnership Project 3GPP and known for example as UMTS terminals are particularly suitable for being used in connection with the present invention;        although reference is made herein to multimedia call or session comprising a video component, this exemplifies only a specific example of content; content as used in the present invention is intended to mean multimedia data of at least one of audio data, video data, image data, text data, and meta data descriptive of attributes of the audio, video, image and/or text data, any combination thereof or even, alternatively or additionally, other data such as, as a further example, program code of an application program to be accessed/downloaded;        method steps likely to be implemented as software code portions and being run using a processor at one of the entities described herein below are software code independent and can be specified using any known or future developed programming language;        method steps and/or devices likely to be implemented as hardware components at one of the entities are hardware independent and can be implemented using any known or future developed hardware technology or any hybrids of these, such as MOS, CMOS, BiCMOS, ECL, TTL, etc, using for example ASIC components or DSP components, as an example;        generally, any method step is suitable to be implemented as software or by hardware without changing the idea of the present invention;        devices or means can be implemented as individual devices or means, but this does not exclude that they are implemented in a distributed fashion throughout the system, as long as the functionality of the device is preserved.        
2. Related Prior Art
In the last years, an increasing extension of communication networks, e.g. of wire based communication networks, such as the Integrated Services Digital Network (ISDN), or wireless communication networks, such as the cdma2000 (code division multiple access) system, cellular 3rd generation (3G) communication networks like the Universal Mobile Telecommunications System (UMTS), cellular 2nd generation (2G) communication networks like the Global System for Mobile communications (GSM), the General Packet Radio System (GPRS), the Enhanced Data Rates for Global Evolutions (EDGE), or other wireless communication system, such as the Wireless Local Area Network (WLAN), took place all over the world. Various organizations, such as the 3rd Generation Partnership Project (3GPP), the International Telecommunication Union (ITU), 3rd Generation Partnership Project 2 (3GPP2), Internet Engineering Task Force (IETF), and the like are working on standards for telecommunication network and multiple access environments.
In general, the system structure of a communication network is such that one party, e.g. a subscriber's terminal device, such as a mobile station, a mobile phone, a fixed phone, a personal computer (PC), a laptop, a personal digital assistant (PDA) or the like, is connected via transceivers and interfaces, such as an air interface, a wired interface or the like, to an access network subsystem. The access network subsystem controls the communication connection to and from the communication equipment and is connected via an interface to a corresponding core or backbone network subsystem. The core (or backbone) network subsystem switches the data transmitted via the communication connection to a destination party, such as another terminal device, a service provider (server/proxy), or another communication network. It is to be noted that the core network subsystem may be connected to a plurality of access network subsystems. Depending on the used communication network, the actual network structure may vary, as known for those skilled in the art and defined in respective specifications, for example, for UMTS, GSM and the like.
Generally, for properly establishing and handling a communication connection between network elements such as the terminal device and another communication equipment or terminal device, a database, a server, etc., one or more intermediate network elements such as control network elements, support nodes or service nodes are involved.
One application area whose importance for current and future communication systems increases comprises multimedia communication services. A multimedia call is a communication where, for example, voice, text, data, video and/or picture are used simultaneously. Multimedia calls generally require the transmission of several different types of data (video, audio, and the like) in parallel, and these data are to be transmitted and received by various different types of communication equipments or network elements, so that it is required that plural communication protocols are negotiated and appropriate parameters for the communication are adjusted.
A current technology to merge the Internet with the cellular telecommunication world is the Internet Protocol (IP) Multimedia Subsystem IMS. IMS is a standardized architecture for operators intending to provide mobile and fixed multimedia services. IMS uses a Voice over IP (VoIP) implementation based on a 3GPP standardized implementation of Session Initiation Protocol (SIP) and runs over the standard Internet Protocol (IP). Both packet-switched and circuit switched communication systems are supported.
The goal is to make available services offered by the Internet nearly everywhere by means of cellular mobile communication systems. IMS is introduced as part of the 3GPP standards since Release 5. As a part of the signalling mechanisms used between the IMS and an user equipment the Session Initiation Protocol (SIP) is used. Details of the structure and procedures executed in IMS are described in the related standards and are commonly known to a person skilled in the art so that a description thereof is omitted herein for the sake of simplicity.
It is expected that current circuit switched networks evolve towards the IMS in the coming years. Thus, for a relatively long period of time both circuit switched (CS) networks and packet switched (PS) networks, like IMS, will be used side by side. Hence, it is necessary to ensure interworking between the both CS and PS systems so that an end user experience is not jeopardized.
However, in case of multimedia calls, in particular of video calls, the situation may be complicated. Such multimedia or video calls are an important feature of newer 3G networks. During a multimedia call, for example, a user has an option to change the media composition of the call, e.g. by abandoning/dropping a video call component or by adding a data transmission component, for example. However, when a multimedia/video call is established and ongoing between, for example, a terminal device associated with an IMS network and a terminal device associated with a circuit switched (CS) network, the CS terminal device being behind a CS leg which is, for example, an ISUP (ISDN (Integrated Services Digital Network) User Part) leg. ISUP, however, does not support swapping between multimedia and speech during an ongoing session/call. In other words, the CS side does not support a change in a media composition of the communication connection which could be instructed from the other side of the connection. Thus, there may arise a situation where one party, such as the IMS terminal/user, may try to change the composition of the session by trying to add or delete a media stream to/from the session. For example, the IMS terminal/user may try to drop the video stream and turn the session/call to a plain speech session/call.
In case an offer to add a media stream is entered, the situation can be handled normally by a corresponding interworking node between the two networks where the respective parties reside, such as a Media Gateway Control Function (MGCF) as the PS/CS interworking node, by turning down the offer. This may be executed by responding with the acceptable media line description. However, in case of an offer to drop a media stream, i.e. the abandonment of a media component of the communication connection, the situation differs. Here, the offer can only be accepted by the other party. In such a PS/CS interworking case (IMS to CS connection) the other party of the PS leg is the interworking node (MGCF) which has a CS video leg, without a capability to drop the video stream, to handle. Therefore, a conflict may arise as there is no possibility to perform a media composition change at the CS side even though it is instructed since it is lying behind the ISUP leg which does not support the media composition change, in particular in case of an abandonment of a media component. On the other hand, the users of the terminal devices involved in the communication connection may feel discomfort or confusion since the communication connection parameters change in an unexpected manner.